Abstract:

One aspect of the present disclosure relates to a method or determining
location(s) at which at least one magnetic article is to be positioned
during a magnetic resonance imaging procedure of at least one subject. A
magnetic field Bo is applied to a region that includes the at least one
subject and does not include the at least one magnetic article. First
magnetic resonance information about the region in response to the
applied magnetic field BO is received. The first magnetic resonance
information relates at least in part to one or more magnetic field
inhomogeneities in the region. Based at least in part on the first
magnetic resonance information, at least one first location proximate the
at least one subject at which at least one paramagnetic article and/or
diamagnetic article is to be positioned is determined, so as to at least
partially compensate for the one or more magnetic field inhomogeneities.

Claims:

1. A method of determining locations at which at least two magnetic
articles are to be positioned during a magnetic resonance imaging
procedure of at least one rodent, the method comprising:A) applying a
magnetic field B0 to a region that includes the at least one rodent
and does not include the at least two articles;B) receiving first
magnetic resonance information about the region in response to the
applied magnetic field B0, the first magnetic resonance information
relating at least in part to one or more magnetic field inhomogeneities
in the region;C) determining, based at least in part on the first
magnetic resonance information, at least one first location proximate the
at least one rodent at which at least one paramagnetic article is to be
positioned so as to at least partially compensate for the one or more
magnetic field inhomogeneities; andD) determining, based at least in part
on the first magnetic resonance information, at least one second location
proximate the at least one rodent at which at least one diamagnetic
article is to be positioned so as to at least partially compensate for
the one or more magnetic field inhomogeneities.

2. The method of claim 1, further comprising:prior to steps C) and D),
determining a plurality of magnetic induction fields U induced in the
region by a plurality of reference magnetic articles each having a
reference magnetic susceptibility, the plurality of reference magnetic
articles being placed at a plurality of different positions N proximate
to the at least one rodent, wherein the steps C) and D) are performed
based at least in part on the first magnetic resonance information and
the plurality of magnetic induction fields U.

3. The method of claim 2, further comprising determining inhomogeneous
magnetic field information T by comparing the static magnetic field
B0 and the first magnetic resonance information.

4. The method of claim 3, further comprising determining, for the
plurality of different positions N proximate to the at least one rodent,
a plurality of scaling values η which, when respectively multiplied
by the reference magnetic susceptibility, represent a plurality of actual
magnetic susceptibilities at the plurality of different positions N that
facilitate compensation of the one or more magnetic field
inhomogeneities.

6. The method of claim 4, wherein i is a first index, j is a second index,
M is a number of positions r within the region, and determining the
plurality of scaling values comprises finding η that minimizes j =
1 M [ T ( r j ) + i = 1 N η i U i
( r j ) ] 2 . ##EQU00003##

7. The method of claim 4, further comprising determining, for each of the
plurality of different positions N corresponding to a non-zero value for
η, an integer number of diamagnetic and/or paramagnetic elements that
equals or most closely approximates a corresponding one of the plurality
of actual magnetic susceptibilities.

8. The method of claim 7, wherein the first location determined in the
step C) is a first position of the N positions corresponding to a first
non-zero value for η, and the second location determined in the step
D) is a second position of the N positions corresponding to a second
non-zero value for η.

9. The method of claim 1, wherein the at least one rodent comprises a
mouse.

10. The method of claim 1, wherein the at least one rodent comprises a
plurality of rodents.

11. The method of claim 1, further comprising:placing the at least one
paramagnetic article in the at least one first location;placing the at
least one diamagnetic article in the at least one second location;
andperforming magnetic resonance imaging of the at least one rodent with
the at least one paramagnetic article in the at least one first location
and the at least one diamagnetic article in the at least one second
location.

12. A magnetic resonance imaging system, comprising:a magnetic field
generator to generate a magnetic field in a region that includes at least
one rodent;at least one support member to support the at least one rodent
in the region in which the magnetic field is generated; andat least one
paramagnetic article and at least one diamagnetic article positioned on
the at least one support member and proximate to the at least one rodent
so as to reduce one or more inhomogeneities of the magnetic field in the
region and proximate to or within the at least one rodent.

13. The magnetic resonance imaging system of claim 12, wherein the at
least one rodent comprises a plurality of rodents.

14. The magnetic resonance imaging system of claim 12, wherein the
magnetic field is static.

15. The magnetic resonance imaging system of claim 12, wherein the support
member has a plurality of fixed positions at which at least one magnetic
article may be placed, and wherein the at least one diamagnetic article
and the at least one paramagnetic article respectively are positioned at
one or more of the plurality of fixed positions.

16. The magnetic resonance imaging system of claim 15, wherein the
plurality of fixed positions are arranged as a grid on the at least one
support member.

17. The magnetic resonance imaging system of claim 15, wherein the at
least one paramagnetic article and the at least one diamagnetic article
have substantially a same shape as one or more of the plurality of fixed
positions.

18. The magnetic resonance imaging system of claim 12, wherein the support
member has a substantially cylindrical shape or a tapered cylindrical
shape.

19. The magnetic resonance imaging system of claim 12, wherein the at
least one support member is configured such that the at least one
paramagnetic article and the at least one diamagnetic article are
positioned within approximately two centimeters of the at least one
rodent.

20. The magnetic resonance imaging system of claim 12, wherein the at
least one support member is configured such that one or more of the at
least one paramagnetic article and the at least one diamagnetic article
are positioned within approximately five millimeters of the at least one
rodent.

21. The system of claim 12, wherein the at least one paramagnetic article
comprises at least one of zirconium and niobium.

22. The system of claim 12, wherein the at least one diamagnetic article
comprises at least one of bismuth and crystalline graphite.

Description:

[0005]Magnetic resonance imaging (MRI) is a technique used frequently in
medical and research applications to produce images of the inside of
subjects such as humans and animals. MRI is based on detecting nuclear
magnetic resonance (NMR) signals, which are electromagnetic waves emitted
by atomic nuclei in response to excitation by an electromagnetic field.
In particular, magnetic resonance (MR) techniques involve detecting NMR
signals produced upon the re-alignment of the nuclear spins of atoms in
the subject's tissue.

[0006]During an MRI procedure, NMR signals emitted from a volume of
interest or from a slice (i.e., a relatively thin region) of the volume
of interest are detected. The detected NMR signals may then be utilized
to produce a two-dimensional (2D) image of the slice. A 2D image of a
slice is composed of pixels, each pixel having an intensity (e.g., a
magnitude or value) that is proportional to the strength of the NMR
signal emitted by a corresponding location in the volume of interest. A
plurality of such 2D images reconstructed from NMR signal data obtained
from successive slices may be stacked together to form a
three-dimensional (3D) image. A 3D image is composed of voxels, each
voxel having an intensity proportional to the strength of the NMR signal
emitted from a corresponding portion of the volume of interest.

[0007]To obtain NMR signals, a static magnetic field B0 is applied to
a region of interest, and nuclei within the region are excited by
applying RF electromagnetic radiation at the Larmor frequency. The Larmor
frequency is the frequency at which nuclear spins process about the axis
of the static magnetic field B0, and is proportional to the strength
of the static magnetic field B0. When applied, the RF
electromagnetic radiation at the Larmor frequency causes the nuclear
spins to change orientation, such that the spins are no longer aligned
with the static magnetic field B0. The nuclear spins then gradually
re-realign with the static magnetic field B0, releasing
electromagnetic energy at the Larmor frequency that is detectable as NMR
signals. Accordingly, the NMR signals contain information that is
significantly dependent on the static magnetic field B0. The NMR
signals may be detected using one or more RF coils sensitive to
electromagnetic changes caused by the NMR signals.

[0008]Inhomogeneities in the applied magnetic field B0 may arise in
various subjects, such as animals and humans, and may be caused by
boundaries, such as tissue-air boundaries which cause disruptions in the
magnetic field B0. Since the Larmor frequency is proportional to the
magnetic field B0, inhomogeneities in the magnetic field B0 may
cause the Larmor frequency to be shifted in some areas. Thus, the RF
electromagnetic radiation emitted from these areas may be shifted from
the expected Larmor frequency, and this electromagnetic radiation may not
be detected as well as electromagnetic radiation emitted at the expected
Larmor frequency. The NMR signals that are detected as a result of such
field inhomogeneities may lead to undesirable artifacts in images
constructed from such NMR signals.

[0009]Conventional techniques for homogenizing the magnetic field B0
include using active or passive compensation components commonly referred
to in the relevant arts as "shims." One example of an active shim is an
electromagnetic coil placed in the static magnetic field B0. The
electromagnetic coil may have a controllable current that induces changes
in the magnetic field around the coil. However, active shims may be
limited to providing relatively coarse, low-order magnetic field
corrections. A passive shim is a piece of magnetic material placed in the
static magnetic field B0 that alters the field around the shim.
However, image artifacts may remain in spite of these conventional
techniques, as they are only partially effective in reducing the magnetic
field B0 inhomogeneities.

SUMMARY

[0010]One aspect of the present disclosure relates to a method of
determining location(s) at which at least one magnetic article is to be
positioned during a magnetic resonance imaging procedure of at least one
subject. A magnetic field B0 is applied to a region that includes
the at least one subject and does not include the at least one magnetic
article. First magnetic resonance information about the region in
response to the applied magnetic field B0 is received. The first
magnetic resonance information relates at least in part to one or more
magnetic field inhomogeneities in the region. Based at least in part on
the first magnetic resonance information, at least one first location
proximate the at least one subject at which at least one paramagnetic
article and/or diamagnetic article is to be positioned is determined, so
as to at least partially compensate for the one or more magnetic field
inhomogeneities. In exemplary implementations, the at least one subject
may include all or only a portion of a human or one or more animals.

[0011]Another aspect of the present disclosure relates to a method of
determining locations at which at least two magnetic articles are to be
positioned during a magnetic resonance imaging procedure of at least one
rodent. A magnetic field B0 is applied to a region that includes the
at least one rodent and does not include the at least two articles. First
magnetic resonance information about the region in response to the
applied magnetic field B0 is received. The first magnetic resonance
information relates at least in part to one or more magnetic field
inhomogeneities in the region. Based at least in part on the first
magnetic resonance information, at least one first location proximate the
at least one rodent at which at least one paramagnetic article is to be
positioned is determined, so as to at least partially compensate for the
one or more magnetic field inhomogeneities. Additionally, based at least
in part on the first magnetic resonance information, at least one second
location proximate the at least one rodent at which at least one
diamagnetic article is to be positioned is determined, so as to at least
partially compensate for the one or more magnetic field inhomogeneities.

[0012]Yet another aspect of the present disclosure relates to a magnetic
resonance imaging system. The magnetic resonance imaging system includes
a magnetic field generator to generate a magnetic field in a region that
includes at least one rodent, and at least one support member to support
the at least one rodent in the region in which the magnetic field is
generated. The magnetic resonance imaging system also includes at least
one paramagnetic article and at least one diamagnetic article positioned
on the at least one support member and proximate to the at least one
rodent so as to reduce one or more inhomogeneities of the magnetic field
in the region and proximate to or within the at least one rodent.

[0013]Yet another aspect of the present disclosure relates to a method of
determining locations at which at least two magnetic articles are to be
positioned during a magnetic resonance imaging procedure of at least one
subject. A magnetic field B0 is applied to a region that includes
the at least one subject and does, not include the at least two articles.
First magnetic resonance information about the region in response to the
applied magnetic field B0 is received. The first magnetic resonance
information relates at least in part to one or more magnetic field
inhomogeneities in the region. Based at least in part on the first
magnetic resonance information, at least one first location proximate the
at least one subject at which at least one paramagnetic article is to be
positioned is determined, so as to at least partially compensate for the
one or more magnetic field inhomogeneities. Additionally, based at least
in part on the first magnetic resonance information, at least one second
location proximate the at least one subject at which at least one
diamagnetic article is to be positioned is determined, so as to at least
partially compensate for the one or more magnetic field inhomogeneities.

[0014]A further aspect of the present disclosure relates to a magnetic
resonance imaging system. The magnetic resonance imaging system includes
a magnetic field generator to generate a magnetic field in a region that
includes at least one subject, and at least one support member to support
the at least one subject in the region in which the magnetic field is
generated. The magnetic resonance imaging system also includes at least
one paramagnetic article and at least one diamagnetic article positioned
on the at least one support member and proximate to the at least one
subject so as to reduce one or more inhomogeneities of the magnetic field
in the region and proximate to or within the at least one subject.

[0016]It should be appreciated that all combinations of the foregoing
concepts and additional concepts discussed in greater detail below are
contemplated as being part of the inventive subject matter disclosed
herein. In particular, all combinations of claimed subject matter
appearing at the end of this disclosure are contemplated as being part of
the inventive subject matter disclosed herein.

BRIEF DESCRIPTION OF DRAWINGS

[0017]The accompanying drawings are not intended to be drawn to scale. In
the drawings, each identical or nearly identical component that is
illustrated in various figures is represented by a like numeral. For
purposes of clarity, not every component may be labeled in every drawing.

[0018]FIGS. 1A-1C illustrate a subject on which magnetic resonance imaging
is desired to be performed and a magnetic resonance imaging apparatus,
according to one embodiment of the present disclosure.

[0019]FIG. 2 illustrates a method of imaging a subject using MRI,
according to one embodiment of the present disclosure.

[0020]FIG. 3 illustrates a method of determining one or more locations at
which magnetic article(s) are to be positioned during a magnetic
resonance imaging procedure, according to one embodiment of the present
disclosure.

[0021]FIG. 4 shows a map illustrating the positions at which magnetic
articles are to be placed and the number of magnetic articles to be
placed at each position for a magnetic resonance imaging procedure
according to one embodiment of the present disclosure.

[0022]FIGS. 5A and 5B illustrate a magnetic resonance image of a subject
and a photograph of the subject, respectively, according to some
embodiments of the present disclosure.

[0023]FIG. 6 illustrates an imaging and computing environment in which
embodiments of the present disclosure may be implemented.

DETAILED DESCRIPTION

[0024]Applicants have recognized and appreciated that the conventional
techniques discussed above for compensating inhomogeneities in a magnetic
field B0 employed for magnetic resonance studies such as MRI may not
provide sufficiently fine corrections in magnetic field magnitude. As a
consequence, magnetic resonance images taken (or other data acquired)
using these prior techniques may still have undesired image artifacts or
errors due to the magnetic field inhomogeneities.

[0025]In view of the foregoing, various embodiments of the present
disclosure are directed to methods and apparatus for improved
compensation of magnetic field inhomogeneities, particularly for MRI
applications and other magnetic resonance studies.

[0026]In some embodiments, a more homogeneous magnetic field is achieved
by placing at least one paramagnetic article and/or at least one
diamagnetic article in the proximity of a region to be imaged (or from
which magnetic resonance information is desired). In general, a
paramagnetic article may add to the nearby magnetic field, and a
diamagnetic article may subtract from the nearby magnetic field. Any
suitable number of diamagnetic and/or paramagnetic articles may be used
to correct magnetic field inhomogeneities, as discussed in further detail
below. Furthermore, a large variety of different configurations (e.g.,
relative positions) of paramagnetic and diamagnetic articles placed in
proximity to a region to be imaged are possible, and an appropriate
configuration of such articles may be determined according to various
methods described herein, to reduce the magnetic field inhomogeneities
and thereby improve the quality of images obtained via an MRI procedure.

[0027]For example, in some embodiments, the position of a plurality of
paramagnetic and/or diamagnetic articles during a magnetic resonance
imaging procedure may be chosen using a magnetic article determination
algorithm. The magnetic article determination algorithm may select
parameters for the type, placement, and/or number of articles to be used
during the magnetic resonance imaging procedure. In some embodiments, the
magnetic article determination algorithm may select these parameters
based on a first "evaluation" MRI procedure of the region without the
plurality of articles, so as to assess the nature of magnetic field
inhomogeneities present. Once such an evaluation MRI is acquired, the
article determination algorithm may determine how to reduce the magnetic
field inhomogeneities by specifying the type, placement and/or number of
articles to be placed in proximity to the region of interest. Then, the
plurality of articles may be placed in the specified position(s)
proximate to the region and an MRI procedure may be performed on the
region with the plurality of articles in place, and the magnetic field
inhomogeneities reduced.

[0028]By way of example, in one embodiment, a researcher may wish to
perform an MRI procedure to image a mouse brain as part of a research
study. However, the brain tissue geometry in the mouse may cause
inhomogeneities in the magnetic field B0. To reduce the
inhomogeneities, one or more diamagnetic and paramagnetic articles may be
placed in appropriate positions near the mouse brain such that the
magnetic field B0 inhomogeneities are effectively reduced. However,
it should be appreciated that the techniques described herein may be used
for imaging any suitable subject, such as mice, other types of rodents
such as rats or guinea pigs, and/or other types of mammals, such as
humans. The techniques described herein may have a variety of
applications such as performing animal research studies and/or performing
medical evaluation of humans. Furthermore, although the present
disclosure describes reducing magnetic field inhomogeneity introduced by
subjects, the concepts disclosed herein can be used to reduce
inhomogeneity from any source, such as from the magnetic field B0 source,
magnetic resonance imaging system surroundings, etc.

[0029]FIG. 1A schematically illustrates a mouse 7 to undergo an MRI
procedure. As discussed above, the mouse's tissue geometry may cause
magnetic field B0 inhomogeneities. FIGS. 1B-1C are diagrams
illustrating top and cross-sectional views, respectively, of an example
of an imaging apparatus 10, according to one embodiment of the present
disclosure, that may be used to facilitate and improve magnetic resonance
imaging of the mouse 7 shown in FIG. 1A. Imaging apparatus 10 includes a
support member 1, inside of which mouse 7 may be placed during an MRI
procedure. The support member is configured so as to facilitate placement
of one or more paramagnetic article(s) 4 and/or one or more diamagnetic
article(s) 5 (examples of which are discussed below) in proximity to the
mouse 7 when the mouse is inside of the support member. In this manner,
the support member 1 provides mechanical support for paramagnetic
article(s) 4 and diamagnetic article(s) 5 during a magnetic resonance
imaging procedure of the mouse.

[0030]More specifically, as illustrated in FIG. 1B, support member 1
includes a plurality of positions 3 at which one or more magnetic
articles (such as paramagnetic article(s) 4 and/or diamagnetic article(s)
5) may be placed. In one exemplary implementation, support member 1 may
have a plurality of axial sections A-F, as illustrated in FIG. 1B, and a
plurality of angular sections 1-16, as illustrated in FIG. 1C (FIG. 1C is
a diagram illustrating a cross section of support member 1 along the line
Z-Z illustrated in FIG. 1B). The plurality of axial sections and angular
sections may form a grid-like pattern (hereinafter referred to simply as
a "grid"), and one or more magnetic article(s) may be placed in any one
of positions 3 on the grid.

[0031]By way of example, one paramagnetic article 4 is shown in FIG. 1B at
position E3, and one diamagnetic article 5 is shown at position C3.
However, it should be appreciated that any suitable configuration,
number, and size of positions may be used for placement of magnetic
articles, and the present disclosure is not limited to the particular
grid illustrated in FIG. 1B.

[0032]Furthermore, although support member 1 is illustrated in FIGS. 1B
and 1C as having a substantially cylindrical shape, it should be
appreciated that any suitable shape for imaging a particular subject may
be chosen according to other implementations pursuant to the concepts
disclosed herein. In some embodiments, the size and shape of support
member 1 may be chosen such that the paramagnetic and/or diamagnetic
article(s) may be positioned as close to the subject (e.g., mouse 7) as
possible, as the magnetic induction field induced by a magnetic article
generally decreases in strength with a greater distance from the article.
In one exemplary implementation, the paramagnetic and/or diamagnetic
article(s) may be positioned within approximately two centimeters of the
surface of the subject, and in another implementation they may be
positioned within approximately five millimeters of the subject. However,
any suitable distance between the subject and the magnetic articles may
be used, as long as the subject and the magnetic articles are in an
effective proximity of one another (e.g., close enough that the field
induced by a given magnetic article remains strong enough to reduce some
magnetic field inhomogeneity in the subject).

[0033]A given paramagnetic article (e.g., paramagnetic article(s) 4) and a
given diamagnetic article (e.g., diamagnetic article(s) 5) contemplated
by the present disclosure may be formed of any suitable material or
combination of materials. As one example, a paramagnetic article may be
formed of zirconium (Zr), which has a magnetic susceptibility X, of
approximately 70×10-6. As another example, a paramagnetic
article may be formed of niobium (Nb), which has a magnetic
susceptibility X, of approximately 225×10-6. Examples of
diamagnetic materials which may be used include bismuth (Bi), which has a
magnetic susceptibility Xs of approximately -160×10-6,
and crystalline graphite, which has a magnetic susceptibility Xs of
from approximately -200×10-6 to -400×10-6. However,
it should be appreciated that any suitable magnetic materials may be
used. In some embodiments, magnetic materials may be chosen that exhibit
particularly strong diamagnetic or paramagnetic properties, which may
reduce the amount of the material needed to induce a strong enough field
to correct for inhomogeneities. In some implementations, materials that
do not exhibit ferromagnetic properties may be preferable for articles
employed to compensate magnetic field inhomogeneities; ferromagnetic
materials may be subjected to a mechanical force when placed in a strong
magnetic field (such as commonly employed in MRI scanners), which may
cause an undesired change in the position of the magnetic article.

[0034]FIG. 2 is a flow chart illustrating a method 20 of performing an MRI
procedure, according to one embodiment of the present disclosure.

[0035]In step 21, magnetic resonance information for a region of interest
(e.g., the head of the mouse 7) is acquired without any magnetic articles
for compensation of field inhomogeneities. For example, mouse 7 may be
placed in the support member 1, without any paramagnetic or diamagnetic
articles placed in the support member, and a first "evaluation" MRI
procedure may be performed to obtain information about the magnetic field
(and/or magnetic flux density) distribution in the region of interest
(e.g., information about the distribution of the magnetic field in the
region of mouse 7 may be obtained). For example, a measured magnetic
field F(r) for each spatial position r in the region, as a result of
applying the static magnetic field B0, may be determined using a
gradient-echo mapping technique, as known in the relevant art. The
measured magnetic field (and/or magnetic flux density) information
obtained from the evaluation MRI procedure the is provided to an article
determination algorithm that uses this information to determine magnetic
article parameters(s) such as placement, number and/or type of magnetic
articles that should be placed on the support member proximate the
subject so as to reduce magnetic field B0 inhomogeneities. One
exemplary article determination algorithm according to the present
disclosure is discussed below in connection with FIG. 3.

[0036]As indicated in FIG. 2, in step 22, via an article determination
algorithm a location may be determined for which at least one
paramagnetic article may be positioned during an MRI procedure to provide
magnetic field compensation. For example, with reference again to FIG.
11B, it may be determined in step 22 that paramagnetic article 4 should
be placed at position E3, as illustrated in FIG. 1B of the support member
1. If a plurality of paramagnetic articles should be used, a plurality of
locations at which the paramagnetic articles should be positioned during
a magnetic resonance imaging procedure may be determined in step 22. As
discussed below in connection with FIG. 3, in one exemplary
implementation one or more magnetic article "maps" may be generated
indicating the locations at which the paramagnetic articles should be
positioned (e.g., on support member 1), one example of which is discussed
below and illustrated in FIG. 4.

[0037]In step 23 of FIG. 2, a location similarly may be determined for
which at least one diamagnetic article may be positioned during an MRI
procedure to provide magnetic field compensation. For example, it may be
determined in step 23 that diamagnetic article 5 should be placed at
position C3, as illustrated in FIG. 1B. If a plurality of diamagnetic
articles should be used, a plurality of locations at which the
diamagnetic articles should be positioned during a magnetic resonance
imaging procedure may be determined in step 23. As discussed above
regarding paramagnetic article(s), one or more magnetic article maps may
be generated showing the locations at which the diamagnetic article(s)
should be positioned (e.g., on support member 1). In some
implementations, steps 22 and 23 may be performed together, and a single
magnetic article map may be generated showing the locations at which both
the paramagnetic and diamagnetic article(s) should be positioned.

[0038]The determined locations may be provided to a human operator or to a
machine so that the magnetic articles may be placed, either manually or
automatically, at positions proximate the subject that correspond to the
determined locations. The placement of the paramagnetic and/or
diamagnetic article(s) may be performed in steps 24 and 25, as indicated
in FIG. 2. For example, the determined locations (e.g., one or more
magnetic article map(s)) may be displayed for the operator (e.g., on a
monitor, printed page, etc.), and the operator may place the magnetic
articles in the appropriate positions proximate the subject (the operator
may affix the magnetic articles to support member 1 in position(s) that
correspond to the determined location(s) that will be in the proximity of
the subject). It should be appreciated that steps 24 and 25 may be
performed in any suitable order, and may be performed together in some
embodiments.

[0039]In step 26 of FIG. 2, an MRI procedure may be performed on the
subject (or a particular region of interest of the subject) with the
magnetic articles in the positions proximate the subject that correspond
to their determined locations on the support member to correct for
magnetic field inhomogeneities. For example, the subject (e.g., mouse 7)
may be placed in a magnetic resonance imaging scanner, with a support
member 1 and magnetic articles placed in their determined positions with
respect to the subject. Accordingly, an MRI procedure may be performed
with a more homogeneous magnetic field.

[0040]FIG. 3 illustrates one exemplary magnetic article determination
algorithm according to the present disclosure for implementing one or
both of the steps 22 and 23 shown in FIG. 2. By way of example, the
support member grid illustrated in FIGS. 1B and 1C has N=96 (e.g.,
6×16) possible positions at which one or more magnetic articles may
be placed. However, it should be appreciated that a support member
according to various embodiments of the present disclosure may be
configured as a grid having virtually any number N of possible magnetic
article positions. In one aspect, greater number of possible magnetic
article positions N may increase the spatial resolution with which
magnetic field corrections may be achieved. In another aspect, a lesser
number of possible magnetic article positions N may increase the speed of
the magnetic article determination algorithm, and decrease the amount of
time needed to place magnetic articles at the determined positions.
Accordingly, an appropriate value for N may be chosen based on these
and/or other criteria.

[0041]Within a region of interest (e.g., within mouse 7), one may define M
volume elements (e.g., voxels), each having a value that represents the
corresponding magnetic field strength at a position in the region
corresponding to the voxel. It should be appreciated that any suitable
number of volume elements M may be used, and M may be chosen depending on
the resolution of an MRI scanner used, the size of the subject, and/or
any other suitable criteria.

[0042]In step 31 of FIG. 3, a "unit response" Ui(rj) may be
determined for each possible magnetic article position i on the support
member 1 grid (e.g., position(s) 3). The unit response Ui(rj)
is the component of the magnetic induction field that results at a
position rj within the subject (simplified to a vector of voxels
rj representing the positions within the subject) in response to
placing a magnetic article of unity susceptibility (Xs=1) at
position i on the grid. By determining the unit response for each
possible magnetic article position, the total response for a plurality of
magnetic articles (at a plurality of magnetic article positions) may be
calculated by adding appropriately scaled individual unit responses from
each position. The term "response" refers to the magnetic field (and/or
flux density) change that would result at positions rj of the region
of interest by adding a particular magnetic article at a particular
position i on the grid. As discussed below, the magnetic article
determination algorithm according to one embodiment may determine the
type and number of magnetic articles to be placed at each position by
optimizing the scaling ηi of the unit response Ui(rj)
at for each possible magnetic article position such that the magnetic
field inhomogeneities are minimized.

[0043]Each unit response Ui(rj=1, 2, . . . M) corresponding to a
particular grid position i on the support member 1 is a vector having
values that represent the magnetic field (an/or magnetic flux density)
strength for a plurality of positions rj (where j=1, 2, . . . M)
within a region of interest that would result from a magnetic article
having a susceptibility of Xs=1 being placed at position i the grid.
Thus, U(r) is a matrix of size N×M that represents the unit
response for each possible magnetic article position (from i=1, 2, . . .
N) that results at each point (j=1, 2, . . . M) within the region of
interest. The unit responses may be found either empirically, by
performing an MRI for a magnetic article placed at each position, by
calculation, or by simulating the magnetic field that would result from
the placement of an article at such a position, etc. The unit responses
may be found prior to performing step(s) 22 and/or 23 of FIG. 2, and may
be stored for use (e.g., on a computer readable medium) in step(s) 22
and/or 23 of FIG. 2. During step 31 of FIG. 3 the unit responses may be
determined by the magnetic article determination algorithm by accessing
the stored unit responses.

[0044]In step 32 of FIG. 3, magnetic field (and/or magnetic flux density)
information about a region of interest (e.g., mouse 7), without field
compensation may be used to assess the presence of field inhomogeneities.
For example, the measured magnetic field F(rj) within the region of
interest, determined in step 21 of FIG. 2, may be compared to the nominal
magnetic field value B0 (e.g., the nominal magnetic field value B
may be subtracted from the measured magnetic field F(r) to obtain the
unwanted magnetic field inhomogeneity T(r) (e.g., T(r)=F(r)-B0),
which is a vector of M elements (for each position j within the region of
interest).

[0045]In step 33 of FIG. 3, the inhomogeneous magnetic field may be
determined. As one example, the matrix equation T=-U.sub.η may be
solved for the variable η. Any suitable technique may be used to
solve such a matrix equation, as would be readily recognized by one of
ordinary skill in the art. For example, the matrix U may be inverted to
obtain U-1, and the inverse may be multiplied by -T to solve for the
vector η The variable η is a vector of variables ηi that
each represent the desired susceptibility for a magnetic article that
should be placed at each position i on the grid.

[0046]This matrix calculation is a simplified calculation (based on
assumptions discussed below) that may be derived from a least-squares
minimization of the field inhomogeneities. As discussed above, T(r)
represents the magnetic field inhomogeneities caused by the subject.
Magnetic articles(s) placed in the proximity of the subject will generate
an induction field A(rj). The induction field within the region is
then given by the superposition of the magnetic field induced by any
paramagnetic or diamagnetic articles at respective positions on the grid,
given by

A ( r ) = i = 1 N η i U i ( r ) ,
##EQU00001##

[0047]where as discussed above, ηi is the desired magnetic
susceptibility determined for each position on the grid of support member
1 and Ui(r) is the induction field unit response for a magnetic
article of unity susceptibility (Xs=1) at a given position on the
grid. If one can create an induction field A(r) that cancels the unwanted
magnetic field inhomogeneity T(r), then the magnetic field inhomogeneity
is eliminated. One approach is to minimize the squared error, which may
be expressed as the following:

[0048]The solutions ηi now represent optimal response scaling
coefficients which may reduce and/or minimize the magnetic field
inhomogeneity. This problem can be farther simplified to the linear
problem T=-Uη, as discussed above. It should be appreciated that the
unit response (for Xs=1) is given as one example for the unit
response, but that any unit response value could be chosen, and the
scaling factors η should be adjusted accordingly if the unit response
is calculated based on a magnetic article of different susceptibility.

[0049]In some embodiments, one or more approximations may be made to
simplify the calculations performed by the magnetic article determination
algorithm discussed above in connection with FIG. 3 so that the
minimization problem may be reduced to a linear problem. The following
three approximations may be used:

[0050](1) In positions not directly adjacent to a magnetic article, the
magnetic induction fields from a magnetic article (or stacks of magnetic
articles) at separate grid positions add in linear superposition.

[0051](2) The amplitude and shape of a magnetic induction field from a
magnetic article is dominated by its own susceptibility and is trivially
perturbed by other nearby materials with much smaller magnetic
susceptibilities (such as tissue). This approximation may be typically
encountered in vivo.

[0052](3) The geometric distribution of an article's induction field does
not change substantially when more than one magnetic article stacked at a
same grid position (in a limited fashion).

[0053]These approximations have been shown to closely approximate the full
magnetostatic field solutions, while saving significant computation time.
In some embodiments, using one or more of these approximations enables
determining the appropriate magnetic article positions within a matter of
seconds. However, it should be appreciated that this exemplary algorithm
is provided merely by way of illustration, and that any suitable
algorithm may be used.

[0054]Once η has been obtained, the actual magnetic articles that
should be placed at each position may be determined. Although each
susceptibility value q may be a continuous quantity, it may be desirable
to use a limited set of materials (e.g., one diamagnetic material and one
paramagnetic material) to achieve the determined susceptibility value or
an approximation thereto.

[0055]In step 34 of FIG. 3, the number and type of paramagnetic and/or
diamagnetic articles that should be placed at the positions i on the grid
may be determined based on the desired susceptibility values ηi
for each position having a nonzero (or approximately nonzero) value
ηi. Given predetermined material types and a predetermined
number of magnetic articles that may be placed at a single position i,
the combination of magnetic elements that most closely approximates the
desired susceptibility value ηi. For example, if the desired
susceptibility value ηi for a position is -320×10-6,
and the materials used are bismuth (X=-160×10-6) and zirconium
(X=70×10-6), then two bismuth elements may be placed at the
position ηi (e.g., 2×-1 60×10-6). As another
example, if the desired susceptibility value ηi for a position
is -80×10-6, then one bismuth and one zirconium element may be
placed at this position to achieve a value of -90×10-6, which
may be relatively close to the desired susceptibility. Thus, the
determined susceptibility value may be approximated by stacking
paramagnetic and/or diamagnetic articles of pre-determined material
types.

[0056]Once the determination is made as to the locations of where the
diamagnetic and paramagnetic articles should be placed, these locations
may be used for positioning the magnetic articles. As one example, the
locations may be displayed for an operator who may then place the
articles into their determined positions. FIG. 4 illustrate an example of
diamagnetic article map 41 showing the determined locations for a
plurality of diamagnetic articles (e.g., made of bismuth). The locations
illustrated in FIG. 4 correspond to the positions illustrated in FIGS. 1B
and 1C. Diamagnetic article map 41 has numbers on the map that indicate
the number of diamagnetic articles determined to be placed at each
position (positions having zero elements are shown as blank). In this
example, a single location may include from 0-3 diamagnetic articles.
However, it should be appreciated that the determined locations may be
presented in any other suitable way, such as using a different type of
visual representation. Similarly, a paramagnetic article map may be
generated that shows the determined locations for the paramagnetic
article(s). In some embodiments, both diamagnetic and paramagnetic
article locations may be shown on the same map.

[0057]FIG. 5A is an example of an image of a mouse (e.g., mouse 7)
acquired pursuant to an MRI procedure according to one embodiment of the
present disclosure. In this embodiment, the magnetic field
inhomogeneities have been reduced, and an image of improved quality is
obtained. This embodiment also illustrates that support member 1 may have
a tapered cylindrical shape, which may help position magnetic articles
close to the subject (e.g., near a mouse's head).

[0058]FIG. 5B is a photograph illustrating a mouse (e.g., mouse 7)
positioned within support member 1, in preparation for imaging. As
illustrated in FIG. 5B, support member 1 has a plurality of magnetic
articles 4 and 5 affixed thereto for correcting magnetic field
inhomogeneities.

[0059]In some embodiments, a plurality of subjects may be imaged together
within a magnetic resonance imaging system. Performing imaging on
multiple subjects at a time may reduce the amount of time needed to
conduct a research study. For example, a plurality of rodents may be
placed in a magnetic resonance imaging system, with each rodent supported
by its own support member having paramagnetic and/or diamagnetic articles
placed thereon.

[0060]If multiple subjects are imaged (either together or separately),
each rodent may have it's own specific magnetic article configuration,
determined in accordance with the techniques described above.
Alternatively, some or all rodents may be provided the same magnetic
article configuration, and not determined on a subject-specific basis.
For example, one "large rodent" magnetic article configuration may be
determined for large mice, and "small rodent" magnetic article
configuration may be determined for small mice. A researcher may be
provided with a kit having a "small rodent module" that includes a
support member sized for small rodents that is preconfigured with
paramagnetic and/or diamagnetic article(s) in a pre-set generic "small
rodent" configuration. Similar modules may be provided in the kit for
rodents that may be used for imaging mice of other sizes. In some
implementations, a kit may be provided having modules that are configured
to fit different sizes of humans (e.g., for imaging the human head).

[0061]FIG. 6 illustrates an imaging and computing system 60 on which
embodiments of the present disclosure may be implemented. System 60
includes a computer 61 that may be coupled to a magnetic resonance
imaging system 62. Magnetic resonance imaging system 62 may include
magnetic field generator 64 that generates the magnetic field B0.
Magnetic resonance imaging system 62 may perform a first MRI procedure on
a subject (e.g., mouse 7) to determine information about the magnetic
field B0, such as the magnetic field B0 distribution in the
region of the subject (e.g., without the magnetic articles in place for
field compensation). The information about the magnetic field B0 may
be provided to computer 61 and/or any other suitable device that
determines the locations at which the magnetic articles should be placed
to correct for the magnetic field B0 inhomogeneities.

[0062]In some embodiments, a magnetic article determination algorithm
(e.g., as discussed above in connection with FIG. 3) may be implemented
on computer 61. Computer 61 may receive magnetic resonance information
about the magnetic field B0 from magnetic resonance imaging system
62, and the determination algorithm may use the magnetic resonance
information to determine the locations at which magnetic articles should
be positioned. However, it should be appreciated that any suitable device
or combination of devices may determine the locations, as the techniques
described herein are not limited to being performed by any particular
hardware or software. Computer readable instructions for performing the
methods described herein may be stored on a computer readable medium 63
in any suitable form. Any type of computer readable media may be used,
such as volatile or non-volatile memory, a magnetic disk, an optical disk
such as a CD-ROM, etc.

[0063]This invention is not limited in its application to the details of
construction and the arrangement of components set forth in the foregoing
description or illustrated in the drawings. The invention is capable of
other embodiments and of being practiced or of being carried out in
various ways. Also, the phraseology and terminology used herein is for
the purpose of description and should not be regarded as limiting. The
use of "including," "comprising," or "having," "containing," "involving,"
and variations thereof herein, is meant to encompass the items listed
thereafter and equivalents thereof as well as additional items.

[0064]Having thus described several aspects of at least one embodiment of
this invention, it is to be appreciated various alterations,
modifications, and improvements will readily occur to those skilled in
the art. Such alterations, modifications, and improvements are intended
to be part of this disclosure, and are intended to be within the spirit
and scope of the invention. Accordingly, the foregoing description and
drawings are by way of example only.